Power tracking device and power tracking method thereof

ABSTRACT

A power tracking device for a power generation apparatus is provided. A multiplier generates a power level signal according to a current signal and a voltage signal both sensed from the output of the power generation apparatus. A sample-and-hold circuit samples the power level signal according to a sampling clock and generates a current level signal, a peak level signal, and a threshold level signal according to an update signal. A compare circuit compares the current, peak and threshold level signals to generate the update signal and a change signal. A converter performs pulse width modulation (PWM) to generate a PWM signal according to a control signal corresponding to the change signal and converts the output of the power generation apparatus to a load according to the PWM signal.

BACKGROUND OF THE INVENTION

1. Field of the Invention

The invention relates to a power tracking device, and more particularlyto a power tracking device for identifying a maximum power point of apower generation apparatus.

2. Description of the Related Art

Power generation apparatuses, such as photovoltaic power generators,wind power generators, and heat power generators etc., are widely usedin various systems. Conversion efficiency of a power generationapparatus is dependant on the conditions of both the load and energysource of the power generation apparatus. The energy source of a powergeneration apparatus may not be stable (e.g. wind power or solar powerlevels may vary) and a load of a power generation apparatus may alsovary. Thus, dynamic control techniques are applied to power generationapparatuses to obtain maximum conversion efficiency of the powergeneration apparatuses.

FIG. 1 shows a diagram illustrating a relationship between voltage andcurrent of an output power of a power generation apparatus. In FIG. 1,curve S1 represents the current/voltage curve of the output power atlower temperature, curve S2 represents the current/voltage curve of theoutput power at middle temperature, and curve S3 represents thecurrent/voltage curve of the output power at higher temperature. Asshown by the curves S1, S2 or S3, when the value of the current of theoutput power is maximized, the voltage level of the output power tendsis minimized, and vice versa. Furthermore, curve S4 represents theoutput power according to the voltage at lower temperature, curve S5represents the output power according to the voltage at middletemperature, and curve S6 represents the output power according to thevoltage at higher temperature. Thus, the maximum power points of thecurves S4, S5 and S6 are different. Namely, different conditions willaffect the maximum power point for the power generation apparatus, suchas variations in wind force, light density, temperature, load, and soon. In order to obtain the maximum conversion efficiency for a powergeneration apparatus, a tracking technique is required to track thepower level of the power generation apparatus, to identify the maximumpower point and transfer the maximum output power from the powergeneration apparatus to the load.

Typically, a maximum power point for a power generation apparatus may beidentified by measuring and comparing the voltage and the current fromthe power generation apparatus. However, there are many possiblevoltage/current points which may be detected, as shown in the curves ofFIG. 1. Thus, identifying a maximum power point by the measuring andcomparing of voltage/current technique is complex and inefficient.

Furthermore, an analog to digital converter (ADC) is used to sample thevoltage and the current from a power generation apparatus and/or from aload and then process the sampled voltage and the sampled currentdigitally. Next, a digital to analog converter (DAC) is used to convertthe computed digital signals to analog signals, so as to output theanalog signal to a pulse width modulation (PWM) device for powercontrol. While digital processes may be flexible when implementingmaximum power point tracking, however, tracking efficiency thereof islimited by the resolution and conversion speed of the ADC and the DACused, as well as computing power. Meanwhile, if a high resolution, highconversion speed or high computing power ADC and DAC is used, requiredarea increases along with costs.

Therefore, a power tracking device for a power generation apparatus isdesired, which simply, quickly, flexibly, and inexpensively identifies amaximum power point of a power generation apparatus.

BRIEF SUMMARY OF THE INVENTION

A Power tracking device and power tracking method therefore for a powergeneration apparatus are provided. An exemplary embodiment of a powertracking device for a power generation apparatus comprises a multiplier,a sample-and-hold circuit, a compare circuit and a converter. Themultiplier generates a power level signal according to a current signaland a voltage signal both sensed from an output signal of the powergeneration apparatus. The sample-and-hold circuit samples the powerlevel signal according to a sampling clock and generates a current levelsignal indicating a current level of the sampled power level signal, apeak level signal indicating a peak level of a sampled power levelsignal that was previously sampled, and a threshold level signalindicating a threshold level of the sampled power level signal accordingto an update signal. The compare circuit compares the current, peak andthreshold level signals to generate the update signal and a changesignal according to the sampling clock. The converter performs pulsewidth modulation (PWM) to generate a PWM signal according to a controlsignal corresponding to the change signal and converts the output fromthe power generation apparatus to a load according to the PWM signal,wherein the duty cycle of the PWM signal is controlled by the controlsignal.

Furthermore, an exemplary embodiment of a power tracking method for apower generation apparatus is provided. A power level signal is obtainedaccording to a current signal and a voltage signal both sensed from theoutput of the power generation apparatus. The power level signal issampled to generate a current level signal indicating a current level ofthe sampled power level signal, a peak level signal indicating a peaklevel of a sampled power level signal that was previously sampled, and athreshold level signal indicating a threshold level of the sampled powerlevel signal according to an update signal. The current, peak andthreshold level signals are compared to generate the update signal and achange signal according to the sampling clock. The output signal of thepower generation apparatus is converted, to generate a power signal, toa load according to a control signal corresponding to the change signal.

A detailed description is given in the following embodiments withreference to the accompanying drawings.

BRIEF DESCRIPTION OF DRAWINGS

The invention can be more fully understood by reading the subsequentdetailed description and examples with references made to theaccompanying drawings, wherein:

FIG. 1 shows a diagram illustrating a relationship between voltage andcurrent of an output power of a power generation apparatus;

FIG. 2 shows a power tracking device for a power generation apparatusaccording to an embodiment of the invention;

FIG. 3 shows an exemplary schematic illustrating the multiplier of FIG.2 according to an embodiment of the invention;

FIG. 4 shows an exemplary schematic illustrating the sample-and-holdcircuit of FIG. 2 according to an embodiment of the invention;

FIG. 5 shows an exemplary schematic illustrating the compare circuit ofFIG. 2 according to an embodiment of the invention;

FIG. 6 shows a diagram illustrating a variation relationship between thecurrent level signal S_(CL), the peak level signal S_(PL) and thethreshold level signal S_(TH) of FIG. 5;

FIG. 7 shows an exemplary schematic illustrating the control circuitaccording to an embodiment of the invention;

FIG. 8 shows an exemplary schematic illustrating a PWM circuit of theconverter of FIG. 2 according to an embodiment of the invention; and

FIG. 9 shows a power tracking method for a power generation apparatusaccording to an embodiment of the invention.

DETAILED DESCRIPTION OF THE INVENTION

The following description is of the best-contemplated mode of carryingout the invention. This description is made for the purpose ofillustrating the general principles of the invention and should not betaken in a limiting sense. The scope of the invention is best determinedby reference to the appended claims.

FIG. 2 shows a power tracking device 200 for a power generationapparatus 100 according to an embodiment of the invention. The powertracking device 200 comprises a multiplier 210, a current sensor 220, avoltage sensor 230, a sample-and-hold circuit 240, a compare circuit250, a clock generator 260, a control circuit 270 and a converter 280.The current sensor 220 and the voltage sensor 230 are coupled betweenthe power generation apparatus 100 and the multiplier 210. The currentsensor 220 and the voltage sensor 230 sense an output S_(OUT) from thepower generation apparatus 100 to generate a current signal S_(I) and avoltage signal S_(V), and transmit them to the multiplier 210,respectively. The multiplier 210 generates a power level signalS_(POWER) indicating a power level of the output S_(OUT) of the powergeneration apparatus 100 according to the current signal S_(I) and thevoltage signal S_(V). The sample-and-hold circuit 240 samples the powerlevel signal S_(POWER) according to a sampling clock CK1 from the clockgenerator 260 and generates a current level signal S_(CL), a peak levelsignal S_(PL) and a threshold level signal S_(TH), and transmits them tothe compare circuit 250 according to an update signal S_(UD) provided bythe compare circuit 250, wherein the current level signal S_(CL)represents a current level of the sampled power level signal S_(POWER),the peak level signal S_(PL) represents a peak level of the sampledpower level signal S_(POWER) that has appeared before, and the thresholdlevel signal S_(TH) represents a threshold level of the sampled powerlevel signal S_(POWER). The compare circuit 250 compares the currentlevel signal S_(CL), the peak level signal S_(PL) and the thresholdlevel signal S_(TH) to generate the update signal S_(UD), and transmitit to the sample-and-hold circuit 240 and generate a change signalS_(CH), and transmit it to the control circuit 270. The clock generator260 generates the sampling clock CK1, and transmits it to thesample-and-hold circuit 240, generates the compare circuit 250 and alevel change clock CK2, and transmits them to the control circuit 270,and generates a saw-tooth signal CK3, and transmits it to the converter280, wherein the clock generator 260 transmits the saw-tooth signal CK3to the converter 280 for performing pulse width modulation (PWM) togenerate a PWM signal S_(PWM). The control circuit 270 generates acontrol signal S_(CT), and transmits it to the converter 280 accordingto the level change clock CK2 and the change signal S_(CH), so as tocontrol the duty cycle of the PWM signal S_(PWM). The converter 280generates a power signal PWR, and transmits it to a load 300 accordingto the PWM signal S_(PWM). In an embodiment, the converter 280 is aDC/DC converter or a DC/AC converter.

In FIG. 2, the multiplier 210 comprises two logarithm (log) amplifiers212 and 214, an adder 216 and an exponential amplifier 218. Referring toFIG. 3, FIG. 3 shows an exemplary schematic illustrating the multiplier210 according to an embodiment of the invention. As shown in FIG. 2 andFIG. 3, the logarithm amplifier 212 is coupled between the currentsensor 220 and the adder 216, and the logarithm amplifier 214 is coupledbetween the voltage sensor 230 and the adder 216. The logarithmamplifier 212 converts the current signal S_(I) from a linear tologarithmic format to generate a logarithmic signal S_(logI), and thelogarithm amplifier 214 converts the voltage signal S_(V) from a linearto logarithmic format to generate a logarithmic signal S_(logV). Next,the adder 216 sums up the logarithmic signals S_(logI) and S_(logV) togenerate a summation signal S_(sum). The exponential amplifier 218 iscoupled between the adder 216 and the sample-and-hold circuit 240, andis used to convert the summation signal S_(sum) from a logarithmic tolinear format to generate the power level signal S_(POWER).

FIG. 4 shows an exemplary schematic illustrating the sample-and-holdcircuit 240 according to an embodiment of the invention. Thesample-and-hold circuit 240 comprises two switches SW1 and SW2, twocapacitors C₁ and C₂, two amplifiers 242 and 244 and a voltage drop unit246. The switch SW1 is controlled by the sampling clock CK1. Thecapacitor C₁ is charged by the power level signal S_(POWER) when theswitch SW1 is turned on. Thus, the power level signal S_(POWER) issampled. Next, the amplifier 242 generates the current level signalS_(CL) according to the voltage of the capacitor C₁ which is the sampledpower level signal S_(POWER). The switch SW2 is controlled by the updatesignal S_(UD). The capacitor C₂ is charged by the current level signalS_(CL) when the switch SW2 is turned on. Next, the amplifier 244generates the peak level signal S_(PL) according to the voltage of thecapacitor C₂ which is a peak level of a previously sampled power levelsignal S_(POWER). Next, the voltage drop unit 246 generates thethreshold level signal S_(TH) according to the peak level signal S_(PL)and a drop voltage ΔV, wherein the drop voltage ΔV may be determinedaccording to requirements.

FIG. 5 shows an exemplary schematic illustrating the compare circuit 250according to an embodiment of the invention. The compare circuit 250comprises two comparators 252 and 254, a logic unit 256 and a flip-flopregister 258. The comparator 252 compares the peak level signal S_(PL)with the current level signal S_(CL) to generate a signal S1. Thecomparator 254 compares the current level signal S_(CL) with thethreshold level signal S_(TH) to generate a signal S2. The logic unit256 generates the update signal S_(UD) according to the sampling clockCK1 and the signals S1 and S2. The flip-flop register 258 generates thechange signal S_(CH) according to the signal S2. In an embodiment, theflip-flop register 258 may be a J-K type flip-flop register or a T typeflip-flop register. FIG. 6 shows a diagram illustrating a variationrelationship between the current level signal S_(CL), the peak levelsignal S_(PL) and the threshold level signal S_(TH) of FIG. 5. First,the peak level signal S_(PL) and the threshold level signal S_(TH) forma dynamic hysteresis window, wherein the range of the hysteresis windowis determined according to the drop voltage ΔV of the voltage drop unit246 in FIG. 4. Referring to FIG. 5 and FIG. 6 together, at time pointt1, the comparator 252 generates the signal S1 to indicate that thecurrent level signal S_(CL) is larger than the peak level signal S_(PL).Thus, the logic unit 256 transmits the update signal S_(UD) to thesample-and-hold circuit 240, so as to update the peak level signalS_(PL) and the threshold level signal S_(TH). During an update periodT_(UP), the peak level signal S_(PL) and the threshold level signalS_(TH) are constantly updated. Next, at time point t2, the comparator254 generates the signal S2 to indicate that the current level signalS_(CL) is smaller than the threshold level signal S_(TH). Thus, thelogic unit 256 transmits the update signal S_(UD) to the sample-and-holdcircuit 240 and then the sample-and-hold circuit 240 replaces the peaklevel signal S_(PL) with the current level signal S_(CL) and obtains anew threshold level signal S_(TH) according to the replaced peak levelsignal S_(PL) and the drop voltage ΔV. Simultaneously, the flip-flopregister 258 changes a polarity of the change signal S_(CH). In theembodiment, the flip-flop register 258 may maintain the polarity of thechange signal S_(CH) when the current level signal S_(CL) is within thehysteresis window, i.e. the current level signal S_(CL) is larger thanthe threshold level signal S_(TH) and smaller than the peak level signalS_(PL).

FIG. 7 shows an exemplary schematic illustrating the control circuit 270according to an embodiment of the invention. The control circuit 270comprises a logic unit 272, two current sources 274 and 276, anamplifier 278, two switches SW3 and SW4 and a capacitor C₃. The currentsource 274 is coupled to a power supply voltage VDD. The switch SW3 iscoupled between the current source 274 and the switch SW4. The currentsource 276 is coupled between the switch SW4 and a ground GND. The logicunit 272 controls the switch SW3 so that it is turned on according tothe change signal S_(CH) and the level change clock CK2, so as to chargethe capacitor C₃ through the current source 274. In addition, the logicunit 272 controls the switch SW4 so that it is turned on according tothe change signal S_(CH) and the level change clock CK2, so as todischarge the capacitor C₃ through the current source 276. Thus, theamplifier 278 generates the control signal S_(CT) according to a voltageof the capacitor C₃.

FIG. 8 shows an exemplary schematic illustrating a PWM circuit 400 ofthe converter 280 according to an embodiment of the invention. The PWMcircuit 400 comprises a comparator 410 which is used to perform PWM togenerate the PWM signal S_(PWM). The comparator 410 compares thesaw-tooth signal CK3 from the clock generator 260 of FIG. 2 with thecontrol signal S_(CT) to generate the PWM signal S_(PWM), wherein theduty cycle of the PWM signal S_(PWM) is controlled according to thecontrol signal S_(CT). The switching points of the control signal S_(CT)occur at the time points that the current level signal S_(CL) is smallerthan the threshold level signal S_(TH), as shown in time point t2 inFIG. 6. In other words, the control signal S_(CT) is reversed when thepolarity of the change signal S_(CH) changes.

FIG. 9 shows a power tracking method 500 for a power generationapparatus according to an embodiment of the invention. First, in stepS510, an output of the power generation apparatus is sensed to obtain acurrent signal (e.g. S_(I) of FIG. 2) and a voltage signal (e.g. S_(V)of FIG. 2). Next, in step S520, a power level signal (e.g. S_(POWER) ofFIG. 2) is obtained according to the current signal and the voltagesignal by converting the current signal and the voltage signal from alinear to logarithmic format to generate two logarithmic signals,summing up the two logarithmic signals and converting the summed signalfrom a logarithmic to linear format. Next, in step S530, the power levelsignal is sampled to generate a current level signal (e.g. S_(CL) ofFIG. 2) indicating a power level of the current sampled power levelsignal, a peak level signal (e.g. S_(PL) of FIG. 2) indicating a peakpower level of a previously sampled power level signal, and a thresholdlevel signal (e.g. S_(TH) of FIG. 2) indicating a threshold level of thesampled power level signal according to a sampling clock (e.g. CK1 ofFIG. 2) and an update signal (e.g. S_(UD) of FIG. 2). Next, in stepS540, the current level signal, the peak level signal and the thresholdlevel signal are compared to generate the update signal and a changesignal (e.g. S_(CH) of FIG. 2) according to the sampling clock. Next, instep S550, the output of the power generation apparatus is converted toa load according to a control signal corresponding to the change signal,so as to identify the maximum power point for the power generationapparatus.

While the invention has been described by way of example and in terms ofpreferred embodiment, it is to be understood that the invention is notlimited thereto. Those who are skilled in this technology can still makevarious alterations and modifications without departing from the scopeand spirit of this invention. Therefore, the scope of the presentinvention shall be defined and protected by the following claims andtheir equivalents.

1. A power tracking device for a power generation apparatus, comprising:a multiplier, generating a power level signal according to a currentsignal and a voltage signal both sensed from an output signal of thepower generation apparatus; a sample-and-hold circuit, sampling thepower level signal according to a sampling clock and generating acurrent level signal indicating a current level of the sampled powerlevel signal, a peak level signal indicating a peak level of a sampledpower level signal that was previously sampled and a threshold levelsignal indicating a threshold level of the sampled power level signalaccording to an update signal; a compare circuit, comparing the current,peak and threshold level signals to generate the update signal and achange signal according to the sampling clock; and a control circuitcoupled to the compare circuit, generating a control signal according tothe change signal and a level change clock.
 2. The power tracking deviceas claimed in claim 1, further comprising: a clock generator, generatingthe sampling clock and the level change clock; and a converter,performing pulse width modulation (PWM) to generate a PWM signalaccording to the control signal corresponding to the change signal andconverting the output signal of the power generation apparatus, togenerate a power signal, to a load according to the PWM signal, whereinthe duty cycle of the PWM signal is controlled by the control signal. 3.The power tracking device as claimed in claim 2, wherein the converteris a DC/DC converter or a DC/AC converter.
 4. The power tracking deviceas claimed in claim 1, further comprising: a current sensor coupledbetween the power generation apparatus and the multiplier, sensing theoutput signal of the power generation apparatus to generate the currentsignal; and a voltage sensor coupled between the power generationapparatus and the multiplier, sensing the output signal of the powergeneration apparatus to generate the voltage signal.
 5. The powertracking device as claimed in claim 4, wherein the multiplier comprises:a first logarithm amplifier coupled to the current sensor, convertingthe current signal from a linear to logarithmic format to generate afirst logarithmic signal; a second logarithm amplifier coupled to thevoltage sensor, converting the voltage signal from a linear tologarithmic format to generate a second logarithmic signal; an adder,summing up the first and second logarithmic signals to generate asummation signal; and an exponential amplifier, converting the summationsignal from a logarithmic to linear format to generate the power levelsignal.
 6. The power tracking device as claimed in claim 1, wherein thecompare circuit generates the update signal and transmits it to thesample-and-hold circuit according to the sampling clock when the currentlevel signal is larger than the peak level signal, and thesample-and-hold circuit updates the peak level signal and the thresholdlevel signal according to the update signal.
 7. The power trackingdevice as claimed in claim 1, wherein the compare circuit generates theupdate signal and transmits it to the sample-and-hold circuit accordingto the sampling clock and changes a polarity of the change signal whenthe current level signal is smaller than the threshold level signal, andthe sample-and-hold circuit replaces the peak level signal with thecurrent level signal according to the update signal.
 8. The powertracking device as claimed in claim 7, wherein the compare circuit holdsthe polarity of the change signal when the current level signal islarger than the threshold level signal and smaller than the peak levelsignal.
 9. A power tracking method for a power generation apparatus,comprising: obtaining a power level signal according to a current signaland a voltage signal both sensed from the output of the power generationapparatus; sampling the power level signal according to a sampling clockand generating a current level signal indicating a current level of thesampled power level signal, a peak level signal indicating a peak levelof a sampled power level signal that was previously sampled and athreshold level signal indicating a threshold level of the sampled powerlevel signal according to an update signal; comparing the current, peakand threshold level signals to generate the update signal and a changesignal according to the sampling clock; and converting the output of thepower generation apparatus, to a load according to a control signalcorresponding to the change signal.
 10. The power tracking method asclaimed in claim 9, further comprising: obtaining the sampling clock anda level change clock; and obtaining the control signal according to thechange signal and the level change clock.
 11. The power tracking methodas claimed in claim 9, further comprising: sensing the output of thepower generation apparatus to generate the current signal; and sensingthe output of the power generation apparatus to generate the voltagesignal.
 12. The power tracking method as claimed in claim 9, wherein thestep of obtaining the power level signal further comprises: convertingthe current signal from a linear to logarithmic format to generate afirst logarithmic signal; converting the voltage signal from a linear tologarithmic format to generate a second logarithmic signal; summing upthe first and second logarithmic signals to generate a summation signal;and converting the summation signal from a logarithmic to linear formatto generate the power level signal.
 13. The power tracking method asclaimed in claim 9, wherein the step of comparing the current, peak andthreshold level signals further comprises: obtaining the update signalwhen the current level signal is larger than the peak level signal, soas to update the peak level signal and the threshold level signal. 14.The power tracking method as claimed in claim 9, wherein the step ofcomparing the current, peak and threshold level signals furthercomprises: obtaining the update signal and changing a polarity of thechange signal when the current level signal is smaller than thethreshold level signal, so as to replace the peak level signal with thecurrent level signal.
 15. The power tracking method as claimed in claim14, wherein the step of comparing the current, peak and threshold levelsignals further comprises: maintaining the polarity of the change signalwhen the current level signal is larger than the threshold level signaland smaller than the peak level signal.
 16. The power tracking method asclaimed in claim 14, further comprising: reversing the control signalwhen the polarity of the change signal changes.
 17. The power trackingmethod as claimed in claim 9, wherein the step of converting the outputsignal of the power generation apparatus further comprises: performingpulse width modulation to generate a PWM signal, wherein the duty cycleof the PWM signal is controlled by the control signal; and convertingthe output of the power generation apparatus to the load according tothe PWM signal.